Modalities that induce specific differentiation to T cell memory in immune responses are important for vaccine design, but there is a paucity of well characterized molecular pathways useful to target for this purpose. We have shown previously that pentoxifylline (PF), a phosphodiesterase (PDE) inhibitor in common clinical use, enhances the commitment of in vitro alloprimed human T cells to secondary responsiveness, a characteristic crucial for memory T cells, which are key determinants of the longevity of the immune response. We now show that this effect can also be mediated by activation of adenylate cyclase (AC) and involves PDE4, but not PDE3 or PDE7. PF-mediated enhancement of T-cell priming is inhibited by blocking AC, is specifically signaled via cAMP-dependent protein kinase A (PKA) isoform I, and is probably independent of both nuclear factor-κB and the mitogen-activated protein kinase cascade. Furthermore, known pharmacological inhibitors of AC or PKA by themselves cannot block T-cell priming in the absence of PF or rolipram (Rm), and enhancement of priming requires the presence of PF only relatively late during a 4-day priming in vitro (at 48-96 h), suggesting that pharmacological extension of cAMP-mediated signaling can bring about an event critical for T cell commitment to memory. Furthermore, PF and Rm prevent induction of caspase activation and apoptosis in anti-CD3-activated human T cells. Together, our data suggest that PKA-I-mediated signals triggered by prolonging the half-life of cAMP induced during T-cell priming increase survival of activated T cells and enhance memory T cell commitment.
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